A series of studies in the rat have been utilized to ascertain the extent to which testosterone and estradiol can regulate the secretion of FSH and LH in the absence of the testes. The data demonstrate that: 1) testosterone alone at physiologic doses can maintain both gonadotropins within the normal range, 2) that estradiol alone preferentially suppresses LH, but not FSH concentrations and 3) that a monotropic increase in plasma FSH is produced when there is reduced androgen production in association with increased estradiol production. These data provide an alternate explanation for the inhibin hypothesis which suggests that there is a specific inhibitor of FSH secretion arising from the seminiferous tubules.